Mass update "Public item flag" on items in Private Item Class

Summary

We are in the strange position that our Item Classes are all private but the items within the private classes are shown as "Public". We now need to disable the public flag for thousands of items.

Content

Please note that the problem with the public flag only applies to items that were imported through FBDI import.
Any items created using the New Item Request flow are working fine, so these all show as private.
For this problem we raised an SR, requesting a way to mass update the public flag for all the existing items, since doing it per item is a massive task.